This invention relates principally to the purification of 1,1,1,3,3-pentafluoropropane, also designated R-245fa, which has been of particular interest as a replacement for chlorofluorocarbons having similar physical properties, particularly R-113. R-245fa may be prepared by a three-step process involving reaction of carbon tetrachloride, CC14, with vinylidene chloride, CH.sub.2 .dbd.CCl.sub.2, reacting the product with HF to form CF.sub.3 CH.sub.2 CF.sub.2 Cl (R-235fa) and finally, hydrogenation to remove the chlorine atom, as disclosed in pending U.S. application Ser. No. 08/099,676.
It is characteristic of such reactions that many by-products are formed, containing varying numbers of hydrogen, chlorine, and fluorine atoms on C.sub.1 -C.sub.3 compounds. These by-products and the unreacted feed material may be separated by distillation where possible. Some compounds are relatively harmless since their presence does not greatly alter the physical properties for which R-245fa is useful. One by-product which must be removed because of its toxicity is 1-chloro-3,3,3,-trifluoropropene (R-1233zd), although only relatively small amounts are typically present in R-245fa as formed. R-1233zd has a boiling point close to that of R-245fa making them difficult to separate by distillation. After distillation of the crude product, R-1233zd will still be present in amounts from about 300 to 20,000 ppm (wt.). It should be reduced to below about 100 ppm (wt.) according to the specifications of the Panel for Advancement of Fluorocarbon Test (PAFTII). Preferably, the R-1233zd should be reduced to 20 ppm (wt.) and most preferably below about 10 ppm (wt.).
Further improvement in methods of purifying R-245fa, particularly with respect to eliminating R-1233zd, is desired and the present inventors have discovered a means for purification by photochlorination which will be disclosed in detail below.
It is advantageous also to remove other unsaturated byproducts which can be present in the R-245fa reaction product, including, for example, R-1223xd, R-1224zb, R-1224xe, R-1233xf, and the like. By way of specific example, chlorotetrafluoropropene (R-1224), including the various isomers thereof, has a boiling point which is similar to the boiling point of R-1233zd and is present as a by-product in the R-245fa reaction product in amounts like those of R-1233zd. Pursuant to the present invention, the amounts of such by-products can be reduced also to values corresponding to the reduced amounts of R-1233zd.